The present invention relates to a pig iron refining process in which industrially pure oxygen is blown in by means of at least one tuyere located below the surface of the molten metal in a converter.
In known installations, the tuyere is generally in the bottom of the converter and comprises two co-axial ducts, the inner ducts being arranged to convey oxygen whereas the space between the inner duct and the outer duct is designed to convey a fluid, preferably an endothermic-decomposition fluid such as a hydrocarbon liquid or gas, to protect the tuyere and the refractory materials against the action of the pure oxygen.
Input and output parameters are monitored and input parameters are controlled in order to obtain steel of a desired quality. Fixed input parameters include the temperature and composition of the pig iron; variable input parameters include oxygen flow rate and the amount and timing of additions to the molten metal; output parameters include the temperature and composition of the waste gases. The desired quality may be, for example, the content of at least one component of the steel and/or the temperature of the steel. A number of methods of controlling the refining operation, when blowing oxygen through the bottom wall of the converter have already been suggested, for example methods based on material and heat balances and a mathematical model for calculating charges.
Theoretically, such methods permit the desired composition and temperature for the refined metal to be regularly obtained upon turn-down of the converter. However, a certain dispersion of the results thus obtained occurs in practice. The dispersion may be due to a lack of information on the charged materials, for example on the weight or the precise composition of the pig iron, the scrap, or the lime. To remedy such a situation, premature turn-down of the converter is generally provided, the iron content and the temperature of the slag are rapidly measured, and blowing is re-started with or without additions and is continued for the time necessary to obtain the desired composition and temperature of the steel.
The results thus obtained have been found to be satisfactory and dispersions, which generally occur when no premature tilting is practiced, are substantially reduced. However, certain difficulties still exist with regard to safety (preventing overflowing or slopping) and surely obtaining the desired temperature and composition of the steel at the end of the refining operation. Such difficulties may be due to the fact that the blowing conditions are not reproducible.
What is required is a procedure which allows these drawbacks to be eliminated with no need for the converter to be prematurely turned down.
It has unexpectedly been found that, by measuring the intensity of sound emitted by the converter it is possible in particular to detect the moment at which substantial dephosphorization of the molten metal is achieved. We have also been able to experimentally determine the existence of a relationship between the quantity of oxygen blown in from the moment at which dephosphorization is achieved and the iron content in the slag.
It is known that the precise moment of the end of the refining operation corresponds to a definite critical iron content of the slag. In the case of phosphoric pig iron, the iron content of the slag is strictly related to the phosphorus content of the molten metal.
Accordingly, measurement of the intensity of sound from the converter can be used to determine the precise moment when the refining operation is terminated.